1. Field of the Invention
The invention relates to an improvement in a process for industrial application of Knoevenagel synthesis involving the condensation of cyanoacetic acid with ketones, e.g., cyclohexanone and cyclododecanone.
2. Discussion of the Background Knoevenagel synthesis has long been known to the art in the literature. A general summary can be found in Houben Weyl, Vol. 8, p. 450. Here the use of weak bases such as ammonia and primary and secondary amines is recommended as a particularly advantageous method of proceeding. In the same literature it is stated that the decarboxylation of the addition products first obtained is facilitated by basic catalysts. ##STR1##
As an example, the reaction of cyclohexanone with cyanoacetic acid is described, using small quantities of ammonium acetate as a catalyst. The reaction takes place in two steps. The reaction product obtained from the first step must then be distilled carefully, because in the process carbon dioxide is violently evolved.
In Org. Synth. Coll., IV, pp. 234-236, this reaction with small quantities of catalyst and isolation of the intermediate, i.e., the cyclohexenyl cyanoacetic acid, is described precisely and in detail. Here, too, it is stated that decarboxylation occurs before or during distillation and then "very rapidly."
This vigorous generation of gas is not a problem in laboratory work, but stands in the way of industrial application, because such gas generation can lead to dangerous bursting of equipment due to sudden excess pressure. The pressure impact can , of course, be handled by high-pressure equipment, but this adds considerably to the cost.
All known processes for industrial use of Knoevenagel synthesis suffer from the disadvantage that they proceed too violently and unevenly and thus entail considerable technical expense for safety reasons. It would be desirable to develop a process whereby the waste gas could be controlled and whereby the waste gas was produced in an even flow without a great lapse of time, if possible immediately after combining the cyanoacetic acid and the ketone. In other words, the intermediate, in the case of cyclohexanone this would be cyclohexenyl cyanoacetic acid, should decarboxylate immediately.
According to Organikum, 12th Ed., VEB Deutscher Verlag der Wissenschaften, Berlin, 1973, p. 508, decarboxylation of the condensation product can be achieved in one step pursuant to a variation on Knoevenagel's process involving the use of amines such as pyridine in large quantities as a solvent with the additional admixture of piperidine. The use of these amines in large quantities makes the preparation of the reaction product much more costly, and problems of waste disposal arise. It would be a great technical advantage if rapid decarboxylation of the condensation product could be achieved without a basic solvent. Furthermore, there would be great interest in such a process by which Knoevenagel synthesis could be carried out on an industrial scale at low technical cost without a safety risk.